Three-point converters can switch three different DC voltage potentials (+DC, zero, −DC) to an AC connection and thus convert a DC voltage into an AC voltage, or an AC voltage into a DC voltage. The components and connections that are required in order to provide an AC connection between the positive and negative DC connection by switching technology are termed phase leg of a converter. The DC-potentials are generally formed by two capacitors connected in series, so that the connection of both capacitors forms the DC zero potential and the respective other connections of the capacitors form the positive or respectively the negative potential. In order that the respective DC potential can be switched to the output, at least four semiconductor switching elements (active electronic switches, e.g. IGBTs) are necessary, which are switched in series between the positive and the negative potential. Additionally, two further circuit means, for example active or passive electronic switches, switch the DC zero potential to the output. In the prior art a distinction is made between NPC (neutral point clamped) and ANPC (advanced neutral point clamped) topology, wherein passive switches such as diodes (MPC) or active switches such as transistors (ANPC) are used for the connection of the DC zero potential. Three-point converters are frequently used to provide voltages of more than 500 V, in particular also to provide medium voltages of 1 kV to 52 kV. Areas of application include energy supply, such as for example wind turbine generators, but also traction drives and further areas in which large electrical powers have to be converted. The use of three-point converters has two basic advantages compared to 2-point converters, which can provide only two DC-potentials. On the one hand higher voltages can be switched since in each case two semiconductor switching elements are switched in series and thus their reverse biases are added up. On the other hand filters that are required in order to damp the harmonics generated by the switching of the switching elements can be made smaller. On account of the switching procedures of the switching elements interfering voltage peaks can occur, which can be reduced by using connections with minimal inductance in the circuit arrangement. For this purpose flat connecting bars or plates are often used, which are placed as close as mechanically possible next to one another and in which the currents flow in opposite directions. Nevertheless since voltage peaks can still occur, so-called snubbers are often provided directly at the connections of the semiconductor switching elements in order to damp these peaks.
DE 42 32 763 A1 discloses a circuit arrangement of a three-point converter, in which a minimum of parasitic inductances in the commutation circuits is to be achieved by equilibrating the leads to the semiconductor switching elements in the individual phases. In the phase legs all semiconductor switching elements are installed along a straight line on a cooling body and establish electrical contact with one another and the intermediate circuit capacitances via three-layer connecting bars. On account of the three-layer current conduction the interspacing of current-conducting layers is not minimal and can vary, so that there is scope for improvement as regards the suppression of parasitic inductances. Furthermore a more compact construction of the converter appears possible, so that this can be optimised for the arrangement in an electrical enclosure.